One guys opinion, if you no she wasn't standing at say between 8 am and noon you should be fine at 6 am. The thing is each cow is differant I have one cow that only stands 1 to 2 hours and one that will stand for ever. My point is the closer you are to the 12 hour mark the better chance you have of not going over the18 hour mark. I get up and breed mine 12 hours after strong standing heat no matter what time it is, not that you have to that's just my way. If your certain of when she started to stand you should have 18 hours. good luck and God bless